Synthetic polymers that are used in rubber compositions and that undergo strain-induced crystallization provide advantageous properties including, for example, tensile strength and abrasion resistance in the rubber composition. Polydienes with a high cis-1,4-linkage content, which exhibit the increased ability to undergo strain-induced crystallization, have been advantageously employed in tires, particularly in the tire components that benefit from such properties such as the sidewall and tread components of tires.
Lanthanide-based catalyst systems comprising a lanthanide compound, an alkylating agent, and a halogen source are useful for polymerizing conjugated diene monomers to produce polydienes having a high cis-1,4-linkage content. The polydienes with the high cis-1,4-linkage content produced with such catalyst systems have a linear backbone structure, exhibit good green strength, and have excellent viscoelastic properties. The linear backbone structure, in turn, is believed to provide excellent properties directed to improved tensile strength, increased abrasion resistance, lower hysteresis, and improved fatigue resistance in the rubber compositions formed with these polydienes. Thus, the polydienes having the high cis-1,4-linkage content produced using lanthanide-based catalyst systems are desirable for use in tires and tire components such as sidewalls and treads.
The polymerization processes disclosed herein provide certain advantages and improvements to the use of lanthanide-based catalyst systems to produce polydienes having a high cis-1,4-linkage content. As discussed herein, the present disclosure relates to using a thiol compound in combination with a lanthanide-based catalyst system to produce polydienes having a higher cis-1,4-linkage content as compared to polydienes prepared under the same polymerization conditions but without the thiol compound.